Magnetic Resonance Imaging (“MRI”) is a medical imaging technique used to visualize tissues and other internal structures of the body. Components of the tissue such as water, fat, and blood each react to magnetization differently, thus these components can vary in their brightness and intensity in an MRI image. Often, clinicians will seek to use these variations to highlight areas of interest. One common technique is to limit the intensity of fat in an MRI image such that fat appears dark, while non-fat structures appear as shades of gray between black and white.
One of the most commonly used techniques for fat suppression is the Composite RF Pulses technique. This technique uses a series of pulses separated by small delays to create 180° phase shifts between fat and water. The Composite RF Pulses technique provides several benefits over other conventional fat suppression techniques. For example, the Composite RF technique is not dependent on the frequency separation between fat and water. Thus, the technique may be effectively used for low-field MR settings wherein the fat-water separation is minimal. Moreover, the Composite RF Pulses technique is also relatively insensitive to non-uniformity in the B1 field. Thus, the technique may also be used in high-field settings.
One drawback of the Composite RF Pulses technique is that it involves large phase shifts which, in turn, require relatively large time delays between each pulse in the series. This results in large phase errors in the presence of B0 inhomogeneity. Moreover, because B0 inhomogeneity increases with B0 field strength, phase errors also increase with field strength. Thus, the Composite RF Pulses technique will either perform poorly or fail to perform in clinical settings where the B0 field strength is high. For example, the technique may erroneously identify water as fat and vice versa.